Vesicle transport is the general process in eukaryotic cells by which proteins synthesized in the endoplasmic reticulum (ER) are transported via the Golgi network to the various compartments in the cell where they will function. Other proteins are transported to the cell surface by this process where they may be secreted. Such proteins include membrane bound receptors or other membrane proteins, neurotransmitters, hormones, and digestive enzymes. The transport process uses a series of transport vesicles that shuttle a protein from one membrane-bound compartment (donor compartment) to another (acceptor compartment) until the protein reaches its proper destination (Rothman, J. E and F. T Wieland (1996) Science 272:227-34).
Protein transport across the ER involves a process that is similar in bacteria, yeast and mammals (Gorlich, D. et al. (1992) Cell 71: 489-503). In the mammalian system, transport is initiated by the action of a cytoplasmic signal recognition particle (SRP) which recognizes a signal sequence on a growing, nascent polypeptide and binds the polypeptide and its ribosome complex to the ER membrane through an SRP receptor located on the membrane. The signal peptide is cleaved and the ribosome complex, together with the attached polypeptide, becomes membrane bound. The mechanism of the subsequent protein translocation process across the ER membrane is largely unknown. It is believed to involve the transient formation of a protein-conducting channel by transmembrane proteins interacting with one another and with the membrane-bound ribosome (Blobel, G. and B. Dobberstein (1975) J. Cell Biol. 67: 852-62).
Possible proteins involved in the ER translocation process in yeast include SEC61p, SEC62p, and SEC63p. Mutations in the genes encoding these proteins lead to defects in the translocation process. SEC61 may be of particular importance since certain mutations in the gene for this protein inhibit the translocation of all proteins tested (Gorlich et al., supra).
Mammalian homologs of yeast SEC61 (mSEC61) have been identified in dog and rat (Gorlich et al., supra). Mammalian SEC61 is also structurally similar to the bacterial cytoplasmic membrane, translocation protein, SECYp. mSEC61 is found in tight association with membrane-bound ribosomes, an association that is induced by membrane-targeting of nascent polypeptide chains and is weakened by dissociation of the ribosomes into their subunits. These studies indicate that mSEC61 is a component of a postulated protein-conducting channel in the ER membrane, and that nascent polypeptides are transferred from the ribosome to this channel (Gorlich et al., supra).
Another aspect of the protein/vesicle transport process, are interactions that occur at the cell membrane control the transport of proteins out of or into a cell. Key to this part of the process are interactions between the cell membrane and a supporting membrane cytoskeleton based on the protein spectrin. A large family of related proteins called ankyrins participate in this part of the transport process by binding to the membrane skeleton protein spectrin and to a protein in the cell membrane called band 3, a component of the anion channel in the cell membrane. Ankyrins therefore function as a critical link between the cytoskeleton and the cell membrane.
Originally found in association with erythroid cells, ankyrins are also found in other tissues as well (Birkenmeier, C. S. et al. (1993) J. Biol. Chem. 268: 9533-40). Ankyrins are large proteins (.sup..about. 1800 amino acids) containing an N-terminal, 89 kDa domain that binds the cell membrane proteins band 3 and tubulin, a central 62 kDa domain that binds the cytoskeletal proteins spectrin and vimenetin, and a C-terminal, 55 kDa regulatory domain that functions as a modifier of the binding activities of the other two domains. Individual genes for ankyrin are able to produce multiple ankyrin isoforms by various insertions and deletions. These isoforms are of nearly identical size but may have different functions. In addition, smaller transcripts are produced which are missing large regions of the coding sequences from the N-terminal (band 3 binding), and central (spectrin binding) domains. The existence of such a large family of ankyrin proteins and the observation that more than one type of ankyrin may be expressed in the same cell type suggests that ankyrins may have more specialized functions than simply binding the membrane skeleton to the plasma membrane (Birkenmeier et al., supra).
In humans, two isoforms of ankyrin are expressed, alternatively, in developing erythroids and mature erythroids, respectively (Lambert, S. et. al. (1990) Proc. Natl. Acad. Sci. 87: 1730-34). A deficiency in erythroid spectrin and ankyrin has been associated with the hemolytic anemia, hereditary spherocytosis (Coetzer, T. L., et al. (1988) N. Engl. J. Med. 318: 230-34).
The discovery of new vesicle transport related proteins and the polynucleotides encoding these proteins satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of cancer and immune disorders.